The purpose of this study was to determine the sensitivity of dogs with
hyperadrenocorticism to treatment with the adrenocorticolytic agent
mitotane. Specifically, we looked for differences in response to treatment using this
drug in dogs with adrenocortical
tumors (adrenal
tumor hyperadrenocorticism, ATH) vs those with pituitary-dependent
hyperadrenocorticism (PDH). For inclusion in this study, each dog must have had clinical signs, data base laboratory abnormalities, and endocrine screening test results consistent with the diagnosis of
hyperadrenocorticism. Further, each dog had to have been treated for at least 6 months with
mitotane and have histologic evidence for adrenocortical or pituitary
neoplasia (all dogs were necropsied). Thirteen dogs with ATH (8
carcinomas, 5
adenomas) were identified. The ages and
body weights of these 13 dogs were computer-matched to 13 dogs with PDH. All dogs were initially treated with approximately 50 mg of
mitotane/kg/d of
body weight. Reexaminations were performed after 7, 30, 90, and 180 days of treatment. Individual dosages varied widely after the initial 5 to 12 days of treatment. The mean (+/- SD) dose of
mitotane (mg/kg/d) for the first 7 days of treatment was 47.5 +/- 9.4 for dogs with ATH vs 45.7 +/- 11.9 for dogs with PDH. The mean plasma
cortisol concentrations 1 hour after
ACTH administration at the 7-day recheck were significantly higher in dogs with ATH (502 +/- 386 nmol/L) than in dogs with PDH (88 +/- 94 nmol/L).(ABSTRACT TRUNCATED AT 250 WORDS)